1. Field of the Invention
The present invention relates to a thermal flow sensor for fluids including gas and liquid (an anemometer, etc.), and in particular relates to a heater and heat sensor thermal flow sensor, hereinafter called "a heater with heat sensor type flow sensor" which is applicable to the control and measurement of gas flow and liquid flow. The invention is more particularly related to medium flow in an air conditioner or cooling apparatus such as a refrigerator or freezer.
2. Description of the Related Art
Conventionally, there exist various sorts of flow sensors. One of them is the thermal-type sensor. As to the thermal-type flow sensor, there exist also various types. The fundamental one among them is the one in which a heat-emitting body such as heater or heat-emitting resistor grid is installed in the fluid, the variation of the energy absorbed by the fluid is detected by measuring the fluid temperature, and thereby the velocity of the fluid can be measured.
As a concrete example, for instance, the published specifications of Japanese Laid-open Patent Publication No. 60-142268/1985 and Japanese Laid-open Patent Publication No. 61-235726/1986 disclose that a channel is formed on a part of the Si substrate by use of anisotropic etching technology, a bridge is suspended over the channel, and a heat-sensitive part is formed on the bridge so that the heat capacitance of the heat-sensitive part can be decreased and further the heat loss on the substrate and the substrate supporter can be reduced. In such construction, the supplied electric power required for heating can be also reduced.
Furthermore, for instance, there exists a method in which the temperature difference between the fluid and the bridge is made constant, both of two sides of the bridge are heated with same electric power. The temperature difference between the upstream side and the downstream side created due to the unbalance of the heat emission into the fluid is detected, and thereby the flowing velocity can be measured.
In such way, since the representative microbridge type flow sensor can measure velocity of the flow with small electric power and high sensitivity, it is a very superior sensor. However, the sensor is easily affected by the ambient temperature and the same has a property of sensitivity greatly depending on the ambient temperature. Such property is not limited to the above microbridge type flowing velocity sensor. It is a large defect of the sensor, commonly, in the overall heat-sensitive flow sensor.
The main reason thereof is that as a resistor element, platinum is employed in the sensor as a material having a small deterioration rate and a high thermal coefficient of resistance.
For this reason, as shown in FIG. 9 as an example, a bridge circuit 1 is constructed with a thin film heater Rh on the bridge together with a temperature sensor using a thin film resistor Rr for measuring the fluid temperature, and first and second standard resistors R.sub.1 and R.sub.2. Regarding a heater with no heat sensor type flow sensor, the output of the bridge circuit 1 is amplified by an operational amplifier 2 and the voltage applied to the heater Rh is controlled by the above-mentioned amplified output, while there exist various methods of performing temperature compensation of the sensitivity thereof.
However, in order to suppress the temperature increase of the thin film heat sensor Rr due to Joule's heat for measuring the fluid temperature which may become a cause of measurement error, it is necessary to make small the electric current flowing through the heat sensor Rr for measuring the fluid temperature.
For this reason, the resistance value of the first standard resistor R.sub.1 of the bridge circuit 1 is required to be a small value. In order to balance the bridge circuit 1, it is necessary to set the ratio of the thin-film resistors respectively forming the other-side which includes the second standard resistor R.sub.2 and the heater Rh to a large value and therefore, the design constraints are restricted. Furthermore, since there exists the voltage drop due to the second standard resistor R.sub.2, it is necessary for the electric power source 3 to have a high voltage in order to apply a sufficient voltage to the heater Rh. As a result, low-voltage driving turns out to be difficult.
Regarding the above matter, as shown in FIG. 10, according to the heater with heat sensor type thermal flow sensor in which a heat sensor using the thin film resistor Rs for measuring the temperature of the thin film heater Rh is provided, and a bridge circuit 4 including the heat sensor Rs for measuring the temperature of the heater there exists no such problem to be solved. It is a merit of the above-mentioned sensor.
The operation of such heater-heat sensor type flow sensor stands on the large assumption that the temperature of the heater Rh is naturally equal to that of the heat sensor Rs for measuring the heater temperature. However, according to our research or studying in the recent years, it has become apparent that the temperature of the position on the bridge sharply decreases when the measured position goes away from the heater. In other words, the temperature of the heater is not equal to that of the heat sensor which is almost 10 .mu.m apart from the heater. Namely, the above-mentioned large assumption is not applied to the above matter. Furthermore, it has been made apparent also that, when the ambient temperature rises, the temperature difference therebetween tends to become large. Even in the case of the heater-heat sensor type, it is difficult to perform the temperature compensation for the circuit thereof.
For instance, consider the case of the measuring circuit construction as shown in FIG. 10. In the case where the difference between the temperatures of the fluid and the heater Rh is .DELTA.T in a certain fluid temperature, assuming that the resistance values of the respective parts are Rr, Rs, R.sub.1 and R.sub.2 respectively at the fluid temperature and the thermal coefficient of resistance for Rs is .alpha., it is preferable to set the resistance values of the first and second standard resistors R.sub.1 and R.sub.2 so as to satisfy the below equation: EQU R.sub.1 .multidot.Rs (1+.alpha.-.DELTA.T)=R.sub.2 .multidot.Rr
However, since the temperature of the heater Rh is not equal to that of the heater temperature sensor Rs in practice, the measuring circuit construction as shown in FIG. 10 cannot keep constant the difference .DELTA.T between the temperature of the fluid and that of the heater Rh for the fluid temperature variation.
Namely, even in the case of the heater-heat type sensor, the reasons of the temperature dependence of the sensitivity thereof are shown below.
A. Temperature dependence of the difference between the temperature of the heater Rh and that of the fluid temperature sensor Rs; PA1 B. Temperature dependence of .DELTA.T due to the construction of the measuring circuit as shown in FIG. 10; and PA1 C. Temperature dependence of the resistance value.
It is necessary to perform the temperature compensation for compensating the above-listed temperature dependence.
Hereupon, as a trial of compensating the temperature dependence of the sensitivity of such heater-heat sensor type thermal flow velocity sensor with the structure and the material of the sensor chip, for instance, the specification of Japanese Laid-open Patent Publication No. 63-241312/1988 discloses that the heater is made of a material such as nichrome alloy having an abstract value of resistance temperature coefficient not larger than 200 ppm/.degree. C. However, the nichrome is a binary alloy of nickel (Ni) and chrome (Cr), and it is not always easy to form a film of the nichrome alloy as the thin-film resistor in the case of the microbridge type flow sensor. In particular, at the time of mass-production thereof, it is difficult to keep constant the ratio of Ni and Cr in the thin-film resistor. Further, the unevenness between the batches becomes large and the cost for adjustment increases.